This invention relates in general to plasma arc cutting of metallic workpieces. More specifically it relates to a circuit and method for reducing nozzle wear while reliably starting a transferred plasma arc torch, even with a large standoff from a workpiece.
Reliable ignition and transfer of a plasma arc torch has been a significant problem throughout the development of plasma technology for cutting metallic workpieces. It is difficult to start a transferred arc between the electrode and the workpiece directly due to the relatively long distance separating them. Therefore, most plasma cutting systems start with a pilot arc between the electrode and the nozzle, a much shorter distance.
There are two ways to start the pilot arc. One solution has been contact starting, one form of which is described in U.S. Pat. No. 4,791,268. However, the principal starting technique in use today uses a high frequency, high voltage (HFHV) signal coupled to a power line from a D.C. power supply to the torch. The HFHV signal induces a spark discharge in a plasma gas flowing between the electrode and a nozzle, typically in a spiral path. A HFHV generator is usually incorporated in a power supply or in a "console" located remotely from the torch and connected to the torch by a lead set. This general arrangement is shown in a highly simplified schematic form in FIG. 1.
The arc between the electrode and nozzle is a pilot arc and the arc between the electrode and the workpiece is a transferred arc. The gas flow through the nozzle is ionized by the pilot arc so that the electrical resistance between the electrode and the workpiece becomes very small. Using a pilot resistor, a higher voltage is applied across the electrode and the workpiece to induce the arc to transfer to the workpiece after the gap is ionized. The time between starting the pilot arc and transferring to the work is a function of the distance of the torch above the work, the pilot arc current level, and the gas flow rate when the traditional start circuits are used. FIG. 2, described below in greater detail, shows a typical start circuit used in plasma cutting systems.
While this technique seems straightforward in practice, its analysis, execution and control present many difficult and complex problems. For example, at the time of arc ignition, the location of the arc on the electrodes, and its maintenance once it is initially struck depend on many factors that vary, and some of which may be interdependent. The result is that the voltage at which breakdown occurs, and the time at which it occurs, are random events. Some of the factors include the cathode and anode geometries and gap spacing, gas pressures, the type of gas, impurities in the gas, nature of local gas flow around the electrodes (laminar, turbulent, amount of swirl), the materials forming the anode and cathode and their surface condition, the place on the electrode where the arc initiates, the available voltage from the power supply, the transient response of the power supply, and electrode and nozzle wear. Interaction of these variables further complicates an analysis or control of ignition. A change in the arc current varies the gas pressure in the torch and the gas flow rate. Electrode and nozzle wear alter the physical location of the initial arc strike, the arc path over the electrode, and the time for the arc travel. Gas impurities deposit on the electrode and nozzle; these deposits change the physical location of the arc strike and the arc voltage. In turn, any increase in the arc voltage, regardless of its source, reduces the ability of the surge injection circuit to provide an initial arc current and, once the arc is struck, to act as a current source sufficient to build to and sustain a steady-state pilot arc.
The time that the pilot arc remains attached to the nozzle is a function of the standoff distance. The higher the standoff, the longer the time will be. The nozzle orifice can be damaged when the time exceeds a certain amount. However, a high standoff distance is necessary when piercing thick metal, e.g. 1/2 inch or more, in order to protect the nozzle front from the molten metal. The nozzle life is therefore short when thick plates are being cut.
A seemingly straightforward solution is to increase the level of the pilot current. The expectation is that this increase will in turn increase the level of ionized gas between the electrode and the workpiece causing the transfer time to decrease. However, in practice this solution does not work. When the standoff distance is high, the nozzle and the workpiece always share the pilot current for a while. This time of sharing leads to a pilot arc attachment time that results in damage to the nozzle.
Another seemingly straightforward solution is to increase the value of a pilot resistor in the pilot arc circuit so that the voltage between the nozzle and the workpiece become greater. This change does help to push the pilot arc to the workpiece, but there is a practical upper limit on the value of the resistor. For example, if the open circuit voltage of the D.C. power supply we used is about 275 volts, and since the arc between the electrode and the nozzle requires a certain amount of voltage, then the voltage drop available across the pilot resistor is limited. The higher the value of the resistor used, the lower the pilot current will be. For large resistors, given the limited potential, the resulting pilot arc current reaches a level that is insufficient to ionize the gap. As a result, transfer does not occur. With a standard 275 volt (open circuit voltage) D.C. power supply powering a Hypertherm model MAX.RTM.200 torch with conventional starting circuitry, as shown in FIG. 2, when a 6 ohm pilot resistor is used, the pilot arc current is only 20 amps. This current is not large enough to ionize the electrode-workpiece gap.
It is therefore a principal object of this invention to provide apparatus and a method of reliably starting and transferring the arc of a plasma arc cutting torch with a high frequency high voltage signal that reduces nozzle wear even where the torch to workpiece standoff is large.
Another object is to provide the foregoing advantage in a manner that is compatible with known starting circuits and requires no changes in the torch or the physics of the plasma.
Yet another object is to provide a system with the foregoing advantages which has a favorable cost of implementation.